1. Field of the Invention
The present invention relates to a flow sensor and a flow rate measuring method.
2. Description of the Prior Art
A flow sensor 1 having the conventional structure is schematically shown in FIGS. 1 and 2. FIG. 2 is a sectional view taken in line X1—X1 in FIG. 1. In FIG. 1, the heater and the temperature measuring unit are shown in exposed form, while the heater and the temperature measuring unit are shown in the form covered by a protective film 10 or the like in FIG. 2. In this flow sensor 1, a depressed space portion 3 is formed in the upper surface of a silicon substrate 2, an insulating thin film 4 is formed on the upper surface of the silicon substrate 2 in such a manner as to cover the space portion 3, and a thin-film bridge portion 5 is formed over the space portion 3 by a part of the insulating thin film 4. The bridge portion 5 is thermally insulated from the silicon substrate 2 by the space (air) in the space portion 3. A heater 6 is arranged at the central portion on the surface of the bridge portion 5, and temperature measuring units 7, 8 are arranged symmetrically about the heater 6 on both sides thereof. The heat-sensing temperature measuring units 7, 8 are formed of a thin film resistor of an iron-nickel alloy, for example, and is capable of measuring the temperature utilizing the change in resistance value with temperature. The surface of the insulating thin film 4 outside the bridge portion 5 is formed with an ambient temperature measuring resistor 9. Further, the silicon substrate 2 is covered by a protective film 10 in such a manner as to protect the heater 6, the temperature measuring units 7, 8 and the ambient temperature measuring resistor 9.
The flow sensor 1 is arranged in the flow path generating a flow of a fluid (the direction in which the fluid flows is indicated by arrow in FIG. 3) as shown in FIG. 3, and the output of the temperature measuring units 7, 8 is monitored while generating heat by supplying current through the heater 6. Specifically, the ambient temperature measuring resistor 9 measures the ambient temperature Tatm, and the heater 6 is controlled to generate heat at a temperature higher by a predetermined value than the ambient temperature Tatm measured by the ambient temperature measuring resistor 9 regardless of the fluid flow rate. Now, assume the following definitions:
V: Mass flow rate of fluid
Cu0: Heat capacity of temperature measuring unit 7
Cd0: Heat capacity of temperature measuring unit 8
Tu(V): Temperature of temperature measuring unit 7 with fluid mass flow rate V
Td(V): Temperature of temperature measuring unit 8 with fluid mass flow rate V
Qu(V): Energy supplied to temperature measuring unit 7 with mass flow rate V, and
Qd(V): Energy supplied to temperature measuring unit 8 with mass flow rate V.
The energy Qu(V), Qd(V) supplied to the temperature measuring unit 7 or 8 in the case where the fluid mass flow rate (hereinafter simply referred to as the flow rate) is V is defined as the energy [=(energy absorbed as heat)−(energy radiated as heat)] supplied to the temperature measuring unit 7 or 8, as the case may be, until a (quasi) equilibrium is reached from the state in which the temperature of the temperature measuring unit 7 or 8 is equal to the ambient temperature Tatm (when the heater 6 is off, for example) as a starting point to the state in which the heater generates heat at a temperature higher by a predetermined value than the ambient temperature Tatm with the fluid passing through the flow sensor 1 at the flow rate V.
At the windless time when no fluid is flowing (i.e. when V=0), equations (1) and (2) below hold. The temperature difference ΔTu0(0), ΔTd0(0) between the temperature Tu(0), Td(0) of the temperature measuring unit 7, 8 and the ambient temperature Tatm with the flow rate of zero is referred to hereinafter as an offset temperature.ΔTu0(0)≡Tu(0)−Tatm=Qu(0)/Cu0  (1)ΔTd0(0)≡Td(0)−Tatm=Qd(0)/Cd0  (2)
When the wind is blowing, i.e. the fluid is flowing at the flow rate V, on the other hand, equations (3) and (4) below hold.ΔTuV(V)≡Tu(V)−Tatm=Qu(V)/Cu0  (3)ΔTdV(V)≡Td(V)−Tatm=Qd(V)/Cd0  (4)As a difference between equations (3) and (1), the following equation (5) is obtained.ΔTu(V)=[Qu(V)−Qu(0)]/Cu0+ΔTu0(0)  (5)Also, as a difference between equations (4) and (2), the following equation (6) is obtained.ΔTd(V)=[Qd(V)−Qd(0)]/Cd0+ΔTd0(0)  (6)
The energy Qd(V) supplied to the temperature measuring unit 8 when the flow rate is V is expressed by a curve as shown in FIG. 4, for example. Thus, according to equation (5), the relation between the temperature change ΔTd(V) of the temperature measuring unit 8 on downstream side with the ambient temperature and the fluid flow rate V is illustrated as an output characteristic shown in FIG. 5A, as an example, assuming that the heat capacity Cd0 and the offset temperature ΔTd0(0) are known. On the other hand, according to equation (6), the relation between the temperature change ΔTu(V) of the temperature measuring unit 7 on upstream side with the ambient temperature and the fluid flow rate V is illustrated as an output characteristic shown in FIG. 5B, as an example, assuming that the heat capacity Cu0 and the offset temperature ΔTu0(0) are known. The initial output characteristic indicating the temperature change ΔTu(V) and the initial output characteristic indicating the temperature change ΔTd(V) are stored in the memory of the operation processing unit of the flow sensor 1. By calculating the temperature change ΔTd(V) with the ambient temperature from the temperature Td(V) measured by the temperature measuring unit 8 and the ambient temperature Tatm measured by the ambient temperature measuring resistor 9, therefore, the value of the flow rate V can be determined using the initial output characteristic shown in FIG. 5A. In similar fashion, by calculating the temperature change ΔTu(V) with the ambient temperature from the temperature Tu(V) measured by the temperature measuring unit 7 and the ambient temperature Tatm measured by the ambient temperature measuring resistor 9, the value of the flow rate V can be determined using the initial output characteristic shown in FIG. 5B. In this way, the use of one of the initial output characteristics shown in FIGS. 5A and 5B can determine the flow rate V of the fluid from the value of ΔTd(V) or ΔTu(V). As an alternative, the flow rate V is determined from the two output characteristics of FIGS. 5A and 5B, and an average value is calculated.
In an environment where the flow sensor is used, the fluid usually contains dust and dirt. Once dust S attaches to the temperature measuring unit 7 or 8 as shown in FIG. 3, the heat capacity of the temperature measuring unit 7 increases beyond the initial heat capacity Cu0 to Cuc (>Cu0), while the heat capacity of the temperature measuring unit 8 increases beyond the initial heat capacity Cd0 to Cdc (>Cd0). Even in the case where the dust S attaches, the relation between the energy Qu(V), Qd(V) supplied to the temperature measuring unit 7, 8 and the flow rate V is considered to remain substantially unchanged. When the dust S attaches to the temperature measuring unit 7, 8, therefore, the aforementioned equations (5) and (6) become the following equations (7) and (8), respectively.ΔTuc(V)=[Qu(V)−Qu(0)]/Cuc+ΔTuc(0)  (7)ΔTdc(V)=[Qd(V)−Qd(0)]/Cdc+ΔTdc(0)  (8)whereΔTuc(0)=Qu(0)/Cuc  (9)ΔTdc(0)=Qd(0)/Cdc  (10)
Therefore, the relation between the temperature change ΔTd(V) of the downstream temperature measuring unit 8 and the fluid flow rate V is such that according as the heat capacity Cdc of the temperature measuring unit 8 increases with the increase in the amount of dust S attached, the output characteristic indicating the temperature change ΔTd(V) changes downward gradually from the initial output characteristic with a smaller gradient as shown in FIG. 6. Also, in the case where the flow rate V is zero, the offset temperature ΔTdc(0) (=Qd(0)/Cdc) for the flow rate V of zero decreases with the increase in heat capacity Cdc.
The conventional flow sensor 1, however, fails to take into consideration the change in the output characteristic due to the dust attached or the like, and has no correcting means. As a result, assuming that the dust S attaches to the temperature measuring unit 8 and the heat capacity of the temperature measuring unit 8 becomes Cdc, so that the characteristic of the temperature measuring unit 8 changes as indicated by the curve expressed in FIG. 6 as shown below,ΔTd(V)=[Qd(V)−Qd(0)]/Cdc+ΔTdc(0)then, the actual value of the flow rate V for the measurement ΔTd(V)=α becomes β. In the conventional flow sensor 1, however, the flow velocity is determined based on the initial output characteristic stored in the memory, and therefore the flow rate calculated by the flow sensor 1 for ΔTd(V)=α is V=γ as shown in FIG. 6. In this way, the conventional flow sensor 1 poses the problem that the attached dust or the like causes an error between the output flow rate value and the actual flow rate. This problem is similarly encountered by the output characteristic of the temperature measuring unit 7 shown in FIG. 5B.
A method of correcting the zero point of the output characteristic using a block valve is available. In this method, however, only the point where the flow rate is zero is corrected, but the change in the profile of the output characteristic is not corrected. Also, the actual output characteristic is not rectilinear as disclosed in the cited patent publication. As shown in FIG. 7, therefore, even in the case where the offset temperature ΔTdc(0) of the output characteristic D1 after dust is attached is corrected to coincide with the offset temperature ΔTd0(0) stored in the memory and the output characteristic D1 is shifted to the output characteristic D2, the portion hatched in FIG. 7 still remains as an error. Thus, the error cannot be sufficiently corrected.